A mechanism that maintains alternative life histories in a loggerhead sea turtle population

Abstract

Intrapopulation variation in habitat use is commonly seen among mobile animals, yet the mechanisms maintaining it have rarely been researched among untrackable species. To investigate how alternative life histories are maintained in a population of the loggerhead sea turtle (Caretta caretta), cumulative reproductive output was evaluated and compared between small planktivores inhabiting oceanic areas (with water depths > 200 m) and large benthivores inhabiting neritic areas (depths < 200 m) that sympatrically nested at Yakushima Island, Japan, from 1986 to 2011. In total, 362 nesting females sampled in three different years were classified into the two foraging groups based on stable isotope ratios in egg yolks. There were significant differences between the two foraging groups in most recorded life history parameters (clutch size, clutch frequency, breeding frequency, and remigration intervals), with the exception of emergence success. We did not find evidence of life history trade-offs, nor age-related changes in fecundity. Over the 26-year study period, we calculated a 2.4-fold greater reproductive output for neritic foragers than for oceanic ones, accounting for breeding and clutch frequency. Temporal consistencies in stable isotope ratios and remigration intervals within females suggested that female Japanese loggerheads show fidelity to respective foraging habitats throughout the adult stage. The large difference in productivity between the two groups was unlikely to be offset by the difference in survival during the period from aboveground emergence to first reproduction, suggesting that oceanic foragers have a lower level of fitness than neritic ones. Together with an absence of genetic structure between foraging groups, we infer that alternative life histories in a loggerhead turtle population are maintained by a conditional strategy.

Loggerhead turtle (Caretta caretta) offspring size does not vary with maternal alternative foraging behaviors: support for their phenotypic plasticity

“…Although the differential habitat use caused a 2.4-fold greater reproductive output for neritic than oceanic foragers in a Japanese loggerhead population (Hatase et al 2013), the two foragers were not genetically distinct at neutral markers such as the 350-base-pair sequences of the mitochondrial DNA control region and five microsatellite loci (Watanabe et al 2011), suggesting that the alternative phenotypes are maintained by a strategy possibly depending on growth conditions during oceanic early life (Hatase et al 2013). However, alternative phenotypes sometimes have a genetic basis even if neutral markers, due to insufficient resolution, fail to detect genetic structure.…”

“…The Yakushima Sea Turtle Research Group has been conducting ongoing, annual tagging surveys of nesting turtles at this site since 1985 (Yakushima Sea Turtle Research Group 2011; Hatase et al 2013). We identified individual female loggerheads by external plastic tags (MultiFlex P, Caisley, Bocholt, Germany) placed on both the front and rear flippers, and an internal tag (ID-100, Trovan, East Yorkshire, UK) inserted into the left front flipper; alternatively, previously existing tags were used for identification.…”

“…Straight carapace lengths and widths of nesting females were measured with calipers (Mantax Blue, Haglöf, Långsele, Sweden) during oviposition. Because this study site has fewer small oceanic foraging loggerheads than large neritic foragers (the ratio is 1:4; Hatase et al 2013), we selected small females with a straight carapace length and width of <800 and <633 mm, respectively, and large females with a straight carapace length and width of ≥800 and ≥633 mm in order to obtain similar sample sizes between small oceanic and large neritic foragers when classified by stable isotope ratios (Hatase et al 2014). We collected one or two pairs of clutches laid by small and large females per night to ensure they experienced the same incubation environment.…”

“…Analytical precision was ≤0.06 ‰ for δ 13 C and ≤0.13 ‰ for δ 15 N. We classified females producing yolks with a δ 13 C of <−18.0 ‰ and a δ 15 N of <12.0 ‰ as oceanic planktivores and females with a δ 13 C of ≥−18.0 ‰ or a δ 15 N of ≥12.0 ‰ as neritic benthivores (Hatase et al 2013(Hatase et al , 2014 following the findings of a previous study that simultaneously conducted stable isotope analysis and satellite telemetry on the same females (Hatase et al 2002b). Although it is difficult to classify the turtles with isotopic values on and around these borders accurately into the two foraging groups, we assumed that data of misclassified turtles would be offset between the two groups during averaging.…”

“…Because (1) female loggerheads grow little after reaching sexual maturity (mean [±SD] growth rate, 2.5 ± 4.0 mm year −1 : Hatase et al 2004) and (2) foraging habitats reflected in stable isotope ratios and remigration intervals (the intervals between successive nesting years) did not vary with breeding experiences (Hatase et al 2002b(Hatase et al , 2013, this size-related foraging dichotomy does not imply that adults change habitat with age at least within Japanese nesting populations. Rather, they would consistently use either oceanic or neritic habitat throughout the adult stage.…”